A common, variable temperature, conductive heating apparatus for semiconductor wafers, known as a thermal wafer chuck, is shown on p. 625 of Nicollian and Brews' MOS Physics and Technology, published in 1982. The chuck is a thermally conductive brass block through which heater elements are placed. The block is spaced above a floor by thermally insulative ceramic spacers to confine heat to the block. Wafers to be tested are held to the block by a vacuum. Grooves on the top surface of the block distribute the vacuum underneath the wafer, making the pressure on the wafer against the block more uniform. Copper tubing is brazed underneath the block through which a coolant such as tap water flows to quickly cool the block back to room temperature after heating.
Other wafer chucks also use water or other fluid flow for cooling. One type of commercial hot chuck passes a coolant, usually tap water, through a cooling coil which is thermally connected to the underside of the chuck. Similarly, another commercially available device uses flexible tubing attached to the chuck and connected to a tap water supply. Alternately, another commercial device has the chuck equipped with coolant passages. Tap water, chilled bottled gas or shop gas can be connected to the chuck to provide rapid cool-down.
One problem with prior art wafer chucks is that coolant flows only during the cooling phase. During the heating phase, the coolant remains stationary in the coils, tubing or passageways so as not to cool the chuck. Some coolant remains in contact with the block. The thermal mass of the cooling structure soaks up heat, increasing thermal load and slowing down the heating process. Further, this coolant is heated during the heating phase, reducing the temperature differential between the coolant and the block so that during the cooling phase, cooling is slowed until cooler water reaches the cooling tubes.
Additionally, during the heating phase, thermal wafer chucks can reach peak temperatures of 300.degree. C. to 400.degree. C., which is well above the boiling point of water. Steam may be generated in the cooling tubes. Eventually, the coils, tubes or passageways corrode and leak. If tap water is used, minerals may be deposited in the tubes, further decreasing their lifetime. In the case where the chuck has coolant passageways, the entire chuck will eventually need to be replaced.
An object of the invention is to produce a variable temperature, conductive heating and cooling apparatus having faster heating and cooling.